1. Field of Invention
The present invention relates generally to electro-optic polymer waveguide devices and methods for fabricating the same and, more particularly, to techniques for electrode poling of electro-optic polymers to eliminate poling induced optical loss and poling induced damage to electro-optic chromophores.
2. Description of the Related Art
An electro-optic polymer is formed with a host polymer and a chromophore molecule. The microscopic nonlinearity of the chromophores in the host material add up to produce the nonlinear property of the electro-optic polymer. However, when the chromophores are initially dispersed in the host material they are randomly oriented such that the nonlinearity of each molecule is canceled. In order to induce nonlinearity in a polymer film, the chromophores are aligned into desired positions by a poling process of applying an electric field across the electro-optic polymer at a high temperature.
During the poling process, a polymer cap layer is employed to prevent breakdown of the electrodes to ground when they are exposed to air and high voltages are applied. Notwithstanding, during such electrode poling, bleaching of the electro-optic polymer near the electrode polymer interface can occur. The bleaching is due to damage to the chromophores in the polymer. The bleaching typically extends several microns into the polymer and is dependent on the polarity of the electrode. This poling induced bleaching of the electro-optic polymer significantly increases the optical loss at both 1,300 nm and 1,550 nm in buried ridge optical waveguides. The increased loss is due to increased optical scattering and/or to loss of optical confinement in the waveguides. Accordingly, there is a need for a fabrication technique which addresses the problem of poling induced damage to electro-optic chromophores.
The principles of the present invention exploit the inventors"" observation that the above-described bleaching is caused by the presence of oxygen during electrode poling and is likely due to the current flow through the polymer and the high electric fields during poling. When poled in air, oxygen readily diffuses through the polymer cap layer and through the cladding and active layers of the waveguides.
According to the present invention, the problem of poling induced damage to electro-optic chromophores is solved by performing the poling process in an oxygen free environment. By employing the technique of the present invention, no bleaching of the chromophores and no increased optical loss been observed as a result of electrode poling. It has been observed that waveguides poled in a nitrogen atmosphere have higher electro-optic coefficients and lower optical loss at both 1,300 nm and 1,550 nm because the chromophore damage is prevented. Thus, the present invention provides a method for preventing chromophore degradation caused by the current/voltage/oxygen bleaching process.
In an exemplary preferred embodiment of the present invention, the electro-optic polymer waveguide devices are placed in a nitrogen atmosphere for a sufficient amount of time to allow any oxygen already in place to diffuse out. The devices are then raised to the poling temperature and the poling voltages are applied for an amount of time required to align the chromophores. The temperature and poling voltage profiles over time are determined depending upon the polymer material and, more specifically, the thermal stability or glass transition temperature of the host polymer material. During poling the typical electric fields are 50 V/micron and the currents are 3 micro-ampere/cm. sq.
A method for manufacturing an electro-optic polymer waveguide device, in accordance with one embodiment of the present invention, includes the step of: electrode poling an electro-optic polymer material of the device in an oxygen-free environment. In a preferred embodiment, the oxygen-free environment comprises nitrogen, argon or helium.
An electro-optic polymer waveguide device, in accordance with another embodiment of the present invention, is manufactured according to the above-described method.
A method for manufacturing an electro-optic polymer waveguide device, in accordance with another embodiment of the present invention, includes the steps of: providing a substrate; providing a bottom electrode on top of the substrate; providing a lower cladding on top of the bottom electrode; providing a core layer of electro-optic polymer material on top of the lower cladding; removing portions of the core layer to form a waveguide structure; providing an upper cladding on top of the waveguide structure; providing an electrode structure on top of the upper cladding; providing a polymer cap layer on top of the electrode structure; and electrode poling the electro-optic polymer material in an oxygen-free environment.
In a preferred embodiment, the method also includes the steps of: removing the polymer cap layer and the electrode structure; providing a strip line electrode structure on top of the upper cladding; and electroplating the strip line electrode structure.
In another preferred embodiment, the step of removing portions of the core layer to form a waveguide structure includes the steps of: providing a protection layer on top of the core layer; patterning waveguide lines on the protection layer; and etching the protection layer to form the waveguide structure. An exemplary preferred protection layer has a thickness of less than 1 xcexcm which allows for better control over the rib etching process. An exemplary preferred protection layer is made from a polymer solution diluted with methanol and is substantially uniform in thickness.
The above described and many other features and attendant advantages of the present invention will become apparent as the invention becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings.